Improved phase sensitive eddy current defect detector utilizing frequency doubling of detected signal prior to phase detection

ABSTRACT

In an eddy current defect detector of the type wherein a bridge detector energized by an oscillator is used to convert the impedance variation of a detecting coil into an electrical signal in accordance with a defect of an object being examined, there is provided a frequency doubler for doubling the frequency of the output from the bridge detector, means to shift the phase of the output of the oscillator by a predetermined angle and double the frequency thereby producing a reference phase signal, and means for sychronously rectifying the output from the frequency doubler by using the reference phase signal.

United States Patent Mori et al.

111 3,721,896 1March 20, 1973 IMPROVED PHASE SENSITIVE EDDY OTHERPUBLICATIONS Stanford & Fearon; Progress in Nondestructive Testing; Vol.1 Heywood 81. Cooper; London; 1958; pp. 89-93 [75] Inventors: ToshihiroMori, Kohoku-ku, Primary Examine" Robert Corcoran Yokohama; Seigo Ando,Kawaski, Attorney-Robe" Flyml et both of Japan ABSTRACT [73] Assrgnee:Nippon Kokan Kabushlkl Kalsha,

T k japan In an eddy current defect detector of the type wherein abridge detector energized by an oscillator is used to [22] Flled: 1971convert the impedance variation of a detecting coil [21] Appl. No.:211,222 into an electrical signal in accordance with a defect of anobject being examined, there is provided a frequency doubler fordoubling the frequency of the output 5%] ..324gi from the bridgedetector, means to Shift the phase f HI. the output of e.oscilla a prede m ned a g e 1e 0 eare and double the frequency thereby producing areference phase signal, and means for sychronously [56] References cuedrectifying the output from the frequency doubler by UNITED STATESPATENTS using the reference phase signal.

3,535,625 10/1970 Pratt ..324/37 6 Claims, 4 Drawing Figures 2 3 IO N I8I OSCIL- POWER BRIDGE FULL-WAVE SYNCHRONOUS LATOR AMP. DETECTORRECTIFIER DETECTOR PHASE FULL-WAVE BPF SHIFTER RECTIFIER I I I I9 20 24IMPROVED PHASE SENSITIVE EDDY CURRENT DEFECT DETECTOR UTILIZINGFREQUENCY DOUBLING OF DETECTED SIGNAL PRIOR TO PIIASE DETECTION Thisinvention relates to an eddy current defect detector, a type ofnon-destructive testing apparatus, and more particularly to a highlysensitive eddy current defect detector wherein the presence and absenceof a defect in an object to be tested is tested by performing a phaseanalysis by using a synchronous rectifier.

To test for the presence or absence of a defect in a magnetizablearticle, for example an iron or steel pipe, an eddy current defectdetector is generally used wherein the pipe is passed through adetection coil, the impedance variation of the coil due to the presenceof the defect is detected as a phase variation created in an electricalsignal flowing through the detector coil and an output corresponding tothe difference between the phase of the electrical signal and areference phase is derived out from a synchronous rectifier circuit.Since the magnitude of the impedance variation of the detectiori coilcaused by the defect varies dependent upon the condition of the defect,a defective pipe can be detected when the phase difference becomes zero,that is when the output from the synchronous rectifier circuit becomesmaximum, if the detector were preset such that the phase of the outputof the detection coil corresponding to the defect which causes a maximumimpedance variation, the largest crack which might appear on the ironpipe for example would match with the phase of the reference signal.However, defects of iron vary over a wide range, that is from a crackfor which reason the pipe should be rejected, to a slight depression ora surface irregularity for which reason the pipe should not be rejectedas being inferior goods. Since such difference in the degree-of defectscan be detected in terms of the phase difference in the output signal,it is possible to determine the accuracy of detection and directivity ofthe eddy current fault detector by the magnitude of the output from thesynchronous rectifier in response to the magnitude of the phasedifference. However, the relationship between the phase difference andthe output (the maximum value thereof being denoted as unity) from thesynchronous rectifier is generally shown by table 1.

TABLE 1 Phase difference (degree) Output As can be noted from table I,in order to discriminate satisfactory products from inferior products itis desirable that the phase difference between electric signalscorresponding to the satisfactory and inferior products is nearly equalto 90". Actually, however, dependent upon the condition of the defect,the phase difference amounts to from 45 to 60 (when expressed in termsof the output from 0.71 to 0.50) in most cases. In this manner, with theprior art eddy current defect detector, the difference in the magnitudeof the signals representing the inferior products and satisfactoryproducts, respectively is not large, so that the phase anglediscriminating ability of the synchronous rectifier is not sufficientlyhigh with the result that satisfactory products are often misjudged asinferior products especially when the level of the signals representingsatisfactory products is high.

SUMMARY OF THE INVENTION Accordingly, it is an object of this inventionto provide an eddy current defect detector capable of very accuratelydiscriminating phase angle differences to thereby discriminate inferiorproducts from satisfactory products. Another object of this invention isto provide an improved eddy current defect detector having an improvedstability and linearity.

According to this invention there is provided an eddy current defectdetector which has a high phase angle discriminating ability comprisinga detecting signal oscillator, a bridge detector constructed to vary thephase of the detecting signal from the oscillator in accordance with adefect in an object being examined, a frequency doubler for doubling thefrequency of the output signal from the bridge detector, means to shiftthe phase of the detecting signal by a predetermined angle and doublethe frequency of the detecting signal to thereby form a reference phasesignal, and means for synchronously rectifying the output from thefrequency doubler by using the reference phase signal.

The present invention can be more fully understood from the followingdetailed description when taken in conjunction with the appendeddrawing, in which:

FIG. 1 shows a connection diagram of one example of the eddy currentdefect detector embodying theinvention;

FIG. 2 shows one example of the bridge detector shown in FIG. 1;

FIG. 3: shows a connectiondiagram of the full-wave rectifier and theband-pass filter utilized in the circuit shown in FIG. 1, and

FIG. 4 shows one example of the phase shifter used in the circuit shownin FIG. 1.

Referring now to FIG. 1, the eddy current defect detector shown thereincomprises an oscillator 1 which produces an AC detecting signal having afrequency most suitable for eddy current defect detection, for examplelKI-I, to 30 KI-I,. The detection signal is supplied to the inputterminal of a bridge detector 3 through a power amplifier 2. The bridgedetector 3 functions to vary the phase of the detection signal inaccordance with a defect of the product being examined.

As shown in FIG. 2, the bridge detector 3 comprises a pair of detectingcoils 7 and 8 and a pair of impedances 4 and 5 which are connected in abridge circuit. An iron pipe 6 to be examined is passed throughdetecting coils 7 and 8. When the pipe 6 is made of homogeneous materialand does not contain any defect, the bridge 3 is balanced and no outputis applied to an amplifier 9. However, when the pipe 6 contains one ormore defects the impedance of either one or both of the detecting coils7 and 8 will vary to provide an output to amplifier 9. The phase angleof the bridge- Referring back to FIG. 1, the output from amplifier 9 isrectified by a full-wave rectifier 10, and the output thereof is appliedto a band-pass filter 11 to provide a signal having a frequency twicethefrequency of the rectifier output. I

As shown in FIG. 3, the full-wave rectifier comprises a transformer 12and a pair of diodes l3 and 14 respectively connected to the oppositeterminals of the secondary winding of transformer 12. The rectifieroutput is taken out across the juncture between diodes 13 and 14 and thejuncture between the center tap of the secondary winding and thegrounded terminal of the primary winding and is applied across theprimary winding 15 ofband-pass filter 11.

The harmonics of the output from full-wave rectifier 10 are analyzedaccording to the following equation.

(305ml 2/17 +4/w 1/3 c0527., 1/15 comm (l/35) cosmt In this equation, itis easy to eliminate the fourth and sixth harmonics and the DC componentto thereby extract a cos 2w: component alone. Since the magnitude ofcos2mt is equal to 0.424 X coswt, it is possible to use cos 2w t as ameasuring parameter. Of course, the phase difference at cos 2 ml istwice as large as the phase difference at cosmt.

As shown in FIG. 3, the band-pass filter 11 comprises a primary winding15 and a resonance circuit constituted by a secondary winding 16inductively coupled to primary winding 15 and a parallel capacitor 17.The tuned frequency of the resonance circuit is set to be equal'to twicethe frequency of the output from amplifier 9. The frequency doubledsignal from bandpass filter 11 is applied to a synchronous detector 18.

A portion of the output signal from oscillator l is also applied to aphase shifter 19 which shifts the phase of the oscillator output signalby a predetermined angle. The degree of phase shift is predetermined tobe substantially equal to the degree of phase shift given by bridgedetector 3 to a signal corresponding to a defect of a magnitude whichrenders the product to be defective.

As shownin FIG. 4 phase shifter 19 comprises a capacitor 21 connected toone terminal of the secondary winding of a transformer 20 and a variableresistor 22 connected to the other terminal of the secondary winding.The output of the phase 19 appears across the juncture between capacitor21 and resistor 22 and the juncture between the center tap of thesecondary winding and the grounded terminal of the primary winding. Theamount of phase shift can be varied from 0 to 180. More particularly,when the resistor 22 is adjusted to a resistance value of zero the phaseis shifted 180, whereas when the resistance value is infinity, the phaseshift is zero. Thus, any desired value of phase shift between 0 to l80can be obtained by the adjustment of variable resistor 22. The output ofthe phase shifter 19 is then supplied to a band-pass filter 24 through afull-wave rectifier 23 which have the same construction as band-passfilter 11 and full-wave rectifier 10, respectively. Thus, band-passfilter 24 provides a signal of the reference phase having a frequencytwice that of the output signal from phase shifter 19.

This reference phase signal is applied to synchronous rectifier 18 wherethis signal is used to synchronously rectify the output from band-passfilter 11 to produce an output proportional to the cosine of the phasedifference between two input signals. The synchronous rectifier 18 usedherein is well known in the art. See for example Robert C. McMasterNondestructive Testing Handbook pp. 40.38 to 40.42, published by TheRonald Press Company (I959), New York.

As above described, since the frequencies of the detected signal andreference phase signal are doubled respectively by the action offull-wave rectifiers l0 and 23 and band-pass filters l 1 and 24, theactual phase difference between two inputs to the synchronous rectifier18 is twice that between the outputs of phase shifter 19 and amplifier9. Taking the phase of the output from band-pass filter 24 as thereference, the phase of the output from band-pass filter 1 1 is twice aslarger.

In this manner, the ratio of the output from the synchronous rectifier18 to the input phase difference is greatly improved as shown infollowing table 2. This means that the eddy current defect detectorembodying the invention is highly directive relative to the phasedifference, thereby resulting in an eddy current detector having a highsensitivity to phase differences.

TABLE 2 Phase difference Output of Output of (degrees) prior detectorthis invention It is to be understood that the negative outputsappearing at angles larger than a 45 difference are treated as zero.- i

In the embodiment shown in FIG. 1, it is advantageous to include anoscilloscope in a stage succeeding amplifier 9 and band-pass filter 11to observe the phase difference of the signal of fundamental frequencyor double frequency.

In the circuit shown in FIG. 1, while the output from full-waverectifier 23 is applied to band-pass filter 24; it

is possible to substitute a Schmidt circuit for. the bandpass filter forobtaining a rectangular wave of double frequency, because a rectangularwave is more advantageous than a sine wave as the reference phase signalto the synchronous rectifier 18.

As above described, the invention provides a highly sensitive eddycurrent defect detector by merely adding a full-wave rectifier and aband-pass filter or a Schmidt circuit to the conventional circuit. Thisdoes not cause any substantial increase in the cost, yet it is possibleto improve stability. Moreover the linearity of the operatingcharacteristic can be improved because it is necessary to shift thephase of the reference phase signal by only one half of that required bythe prior art defect detector.

What we claim is:

1. A phase analyzing type eddy current defect detector comprising:

an oscillator;

a bridge detector for varying the phase of the signal from said.oscillator in accordance with a defect in an object being examined;

a first frequency doubler for doubling the frequency of the outputsignal from said bridge detector;

a reference phase signal generating means including means for shiftingthe phase of the detecting signal from said oscillator by apredetermined angle and a reference signal frequency doubling means fordoubling the frequency of the signal from said oscillator to therebyform a phase shifted, frequency doubled, reference phase signal; and

means responsive to said reference phase signal and to the output ofsaid first frequency doubler for synchronously rectifying the outputfrom said frequency doubler by using said reference phase signal.

2. The eddy current defect detector according to claim 1 wherein saidbridge detector comprises first and second arms respectively includingtwo detecting coils juxtaposed along the path of an object beingexamined and third and fourth impedance arms respectively opposing saiddetecting coils.

3. The eddy current defect detector according to claim 1 wherein saidphase shifting means comprises a transformer including a primary windingreceiving an input signal from said oscillator and a secondary winding,a capacitor connected at one side to one terminal of said secondarywinding and a variable resistor connected between the other terminal ofsaid secondary winding and the other side of said capacitor, and whereinthe degree of phase shift is varied by varying said variable resistor.

4. The eddy current defect detector according to claim 1 wherein saidfirst frequency doubler comprises a full-wave rectifier for rectifyingthe output signal from said bridge detector and a band-pass filter forderiving a component having a frequency twice that of the signal fromthe output from said full-wave rectifier.

5. The eddy current defect detector according to claim 1 wherein saidphase shifting means of said reference phase signal generating means iscoupled to the output of said oscillator, and said reference signalfrequency doubling means is coupled to the output of said phase shiftingmeans to provide said reference phase signal.

6. The eddy current defect detector according to claim 1 wherein saidreference signal frequency 20,

doubling means comprises a full-wave rectifier for rectifying the outputsignal from said phase shifter and a band-pass filter for deriving acomponent having a frequency twice that of the oscillator signal.

1. A phase analyzing type eddy current defect detector comprising: anoscillator; a bridge detector for varying the phase of the signal fromsaid oscillator in accordance with a defect in an object being examined;a first frequency doubler for doubling the frequency of the outputsignal from said bridge detector; a reference phase signal generatingmeans including means for shifting the phase of the detecting signalfrom said oscillator by a predetermined angle and a reference signalfrequency doubling means for doubling the frequency of the signal fromsaid oscillator to thereby form a phase shifted, frequency doubled,reference phase signal; and means responsive to said reference phasesignal and to the output of said first frequency doubler forsynchronously rectifying the output from said frequency doubler by usingsaid reference phase signal.
 2. The eddy current defect detectoraccording to claim 1 wherein said bridge detector comprises first andsecond arms respectively including two detecting coils juxtaposed alongthe path of an object being examined and third and fourth impedance armsrespectively opposing said detecting coils.
 3. The eddy current defectdetector according to claim 1 wherein said phase shifting meanscomprises a transformer including a primary winding receiving an inputsignal from said oscillator and a secondary winding, a capacitorconnected at one side to one terminal of said secondary winding and avariable resistor connected between the other terminal of said secondarywinding and the other side of said capacitor, and wherein the degree ofphase shift is varied by varying said variable resistor.
 4. The eddycurrent defect detector according to claim 1 wherein said firstfrequency doubler comprises a full-wave rectifier for rectifying theoutput signal from said bridge detector and a band-pass filter forderiving a component having a frequency twice that of the signal fromthe output from said full-wave rectifier.
 5. The eddy currEnt defectdetector according to claim 1 wherein said phase shifting means of saidreference phase signal generating means is coupled to the output of saidoscillator, and said reference signal frequency doubling means iscoupled to the output of said phase shifting means to provide saidreference phase signal.
 6. The eddy current defect detector according toclaim 1 wherein said reference signal frequency doubling means comprisesa full-wave rectifier for rectifying the output signal from said phaseshifter and a band-pass filter for deriving a component having afrequency twice that of the oscillator signal.